Cytokinins are a class of N6 substituted purine derivative plant hormones that regulate cell division and influence a large number of developmental events, such as shoot development, sink strength, root branching, control of apical dominance in the shoot, leaf development, chloroplast development, and leaf senescence (Mok, et al., (1994) Cytokinins. Chemistry, Action and Function. CRC Press, Boca Raton, Fla., pp. 155-166; Horgan (1984) Advanced Plant Physiology ed. MB., Pitman, London, UK, pp 53-75; and Letham (1994) Annual Review of Plant Physiol 34:163-197). In maize, cytokinins (CK) play an important role in establishing seed size, decreasing tip kernel abortion, and increasing seed set during unfavorable environmental conditions (Cheikh, et al., (1994) Plant Physiol. 106:45-51; Dietrich, et al., (1995) Plant Physiol Biochem 33:327-36). Active cytokinin pools are regulated by rates of synthesis and degradation.
Until recently, roots were believed to be the major site of cytokinin biosynthesis but evidence indicates that others tissues, such as shoot meristems and developing seeds, also have high cytokinin biosynthetic activity. It has been suggested that cytokinins are synthesized in restricted sites where cell proliferation is active. The presence of several AtIPT genes in Arabidopsis and their differential pattern of expression might serve this purpose.
The catabolic enzyme isopentenyl transferase (IPT) directs the synthesis of cytokinins and plays a major role in controlling cytokinin levels in plant tissues. Multiple routes have been proposed for cytokinin biosynthesis. Transfer RNA degradation has been suggested to be a source of cytokinin, because some tRNA molecules contain an isopentenyladenosine (iPA) residue at the site adjacent to the anticodon (Swaminathan, et al., (1977) Biochemistry 16:1355-1360). The modification is catalyzed by tRNA isopentenyl transferase (tRNA IPT; EC 2.5.1.8), which has been identified in various organisms such as Escherichia coli, Saccharomyces cerevisiae, Lactobacillus acidophilus, Homo sapiens, and Zea mays (Bartz, et al., (1972) Biochemie 54:31-39; Kline, et al., (1969) Biochemistry 8:4361-4371; Holtz, et al., (1975) Hoppe-Seyler's Z. Physiol. Chem. 356:1459-1464; Golovko, et al., (2000) Gene 258:85-93; and, Holtz, et al., (1979) Hoppe-Seyler's Z. Physiol. Chem. 359:89-101). However, this pathway is not considered to be the main route for cytokinin synthesis (Chen, et al., (1997) Physiol. Plant 101:665-673 and McGraw, et al., (1995) Plant Hormones, Physiology, Biochemistry and Molecular Biology. Ed. Davies, 98-117, Kluwer Academic Publishers, Dordrecht).
Another possible route of cytokinin formation is de novo biosynthesis of iPMP by adenylate isopentenyl transferase (IPT; EC 2.5.1.27) with dimethylallyl-diphosphate (DMAPP), AMP, ATP, and ADP as substrates. Our current knowledge of cytokinin biosynthesis in plants is largely deduced from studies on a possible analogous system in Agrobacterium tumefaciens. Cells of A. tumefaciens are able to infect certain plant species by inducing tumor formation in host plant tissues (Van Montagu, et al., (1982) Curr Top Microbiol Immunol 96:237-254; Hansen, et al., (1999). Curr Top Microbiol Immunol 240:21-57). To do so, the A. tumefaciens cells synthesize and secrete cytokinins which mediate the transformation of normal host plant tissues into tumors or calli. This process is facilitated by the A. tumefaciens tumor-inducing plasmid which contains genes encoding the necessary enzyme and regulators for cytokinin biosynthesis. Biochemical and genetic studies revealed that Gene 4 of the tumor-inducing plasmid encodes an isopentenyl transferase (IPT), which converts AMP and DMAPP into isopentenyladenosine-5′-monophosphate (iPMP), the active form of cytokinins (Akiyoshi, et al., (1984) Proc. Natl. Acad. Sci. USA 81:5994-5998). Overexpression of the Agrobacterium ipt gene in a variety of transgenic plants has been shown to cause an increased level of cytokinins and elicit typical cytokinin responses in the host plant (Hansen, et al., (1999) Curr Top Microbiol Immunol 240:21-57). Therefore, it has been postulated that plant cells use machinery similar to that of A. tumefaciens cells for cytokinin biosynthesis. Arabidopsis IPT homologs have recently been identified in Arabidopsis and Petunia (Takei, et al., (2001) J. Biol. Chem. 276:26405-26410 and Kakimoto (2001) Plant Cell Physiol. 42:677-685). Overexpression of the Arabidopsis IPT homologs in plants elevated cytokinin levels and elicited typical cytokinin responses in planta and under tissue culture conditions (Kakimoto (2001) Plant Cell Physiol. 42:677-685).
Arabidopsis ipt genes are members of a small multigene family of nine different genes, two of which code for tRNA isopentenyl transferases, and seven of which encode a gene product with a cytokinin biosynthetic function. Biochemical analysis of the recombinant AtIPT4 protein showed that, in contrast to the bacterial enzyme, the Arabidopsis enzyme uses ATP as a substrate instead of AMP. Another plant IPT gene (Sho) was identified in Petunia hybrida using an activation tagging strategy (Zubko, et al., (2002) The Plant Journal 29:797-808).
In view of the influence of cytokinins on a wide variety of plant developmental processes, including root architecture, shoot and leaf development, and seed set, the ability to manipulate cytokinin levels in higher plant cells, and thereby drastically effect plant growth and productivity, offers significant commercial value (Mok, et al., (1994) Cytokinins. Chemistry, Action and Function. CRC Press, Boca Raton, Fla., pp. 155-166).